In an image-forming process performed in a conventional image-forming device such as a laser printer, a paper-feeding means feeds a sheet of recording medium to a conveying means, and the conveying means conveys the sheet between a transfer roller and a photosensitive drum where a toner image is transferred onto the sheet of recording medium. Subsequently, the toner is heated and melted as the sheet passes between a heating roller and a pressure roller, thereby fixing the toner image to the sheet of recording medium. In order to allocate sufficient time for developing print data and to prevent paper jams when feeding sheets of the recording medium consecutively, the image-forming device opens a prescribed gap (30 mm, for example) between a preceding sheet and a succeeding sheet. Further, since the frictional resistance against the sheet of recording medium on a conveying path differs based on the sheet thickness, this difference will cause a slight variance in the conveying speed and the pressure applied by the pressure roller. Hence, in order to form images of uniform quality on the recording medium, conventional image-forming devices adjust various image-forming conditions, such as the developing bias, transfer bias, and fixing temperature, based on the thickness of the sheet, as disclosed in Japanese unexamined patent application publications Nos. 2003-223022 and HEI-11-49388.
As shown in FIG. 12, a conventional image-forming device typically has a sheet sensor 202 for detecting the trailing edge of a sheet 201 that is fed by a feeding means (not shown) and conveyed by a conveying means (not shown). When the sheets 201 of recording medium are fed consecutively, the sheet sensor 202 detects the trailing edges of the sheets 201, and the image-forming device adjusts the gap between conveyed sheets 201 by controlling the timing at which each sheet 201 is fed from the feeding means to the conveying means based on the detection results.
The sheet sensor 202 is disposed downstream of the feeding means. In order to keep costs down, the sheet sensor 202 includes an actuator 203 and a detector 204. The actuator 203 is capable of pivoting in a direction indicated by arrows N in FIG. 12 and has a front end that protrudes into a conveying path of the sheet 201. The detector 204 is for detecting the rear end of the actuator 203. When the sheet 201 contacts the front end of the actuator 203, the actuator 203 pivots clockwise in FIG. 12. At this time, the detector 204 switches from an OFF state to an ON state, effectively detecting the leading edge of the sheet 201. After the sheet 201 passes over the actuator 203, the actuator 203 returns to its original position by pivoting counterclockwise in FIG. 12. At this time, the detector 204 changes from the ON state to the OFF state, effectively detecting the trailing edge of the sheet 201.
An image forming device further includes a fixing device having a heating roller and a pressure roller. When the heating roller and the pressure roller apply heat and pressure to the sheet 201 in order to fix an image on the sheet 201, the sheet 201 curls. There is a possibility that the curled sheet 201 may cause a paper jam by catching on some component in the image-forming device while being conveyed from the image-forming position to a discharge position. To resolve this problem, the image-forming device is provided with a discharge sensor having the same structure as the sheet sensor 202 described above disposed between the image-forming position and the discharge position in order to monitor the conveyed state of the sheet 201 based on the ON/OFF state of the discharge sensor.